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  设计模式之观察者模式
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  <p>观察者模式</p>
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<h2 id="观察者模式"><a href="#观察者模式" class="headerlink" title="观察者模式"></a>观察者模式</h2><p>观察者模式( Observer Pattern)，又叫发布-订阅( Publish/ Subscribe)模式、模型-视图( Model/View)模式、源-监听噐( Source/ Listener)模式或从属者( Dependents)模式。定义种一对多的依赖关系，一个主题对象可被多个观察者对象同时监听，使得每当主题对象状态变化时，所有依赖于它的对象都会得到通知并被自动更新，属于行为型模式。</p>
<p>观察者模式的核心是将观察者与被观察者解耦，类似于消息/广播发送的机制联动两者，使被观察者的变动能通知到感兴趣的观察者们，从而做出相应的响应。</p>
<p>UML类图如下</p>
<p><img src="https://tu.dxysun.com/image-20210429234749416-20210429234750.png" alt="image-20210429234749416"></p>
<p>从UML类图中，我们可以看到，观察者模式主要包含三种角色：</p>
<p>抽象主题（Subject）：指被观察的对象（Observable）。该角色是一个抽象类或接口，定义了增加、删除、通知观察者对象的方法；</p>
<p>具体主题（ConcreteSubject）：具体被观察者，当其内状态变化时，会通知已注册的观察者；</p>
<p>抽象观察者（Observer）：定义了响应通知的更新方法；</p>
<p>具体观察者（ConcrereObserver）：在得到状态更新时，会自动做出响应。</p>
<h3 id="观察者模式的应用场景"><a href="#观察者模式的应用场景" class="headerlink" title="观察者模式的应用场景"></a>观察者模式的应用场景</h3><p>在软件系统中，当系统一方行为依赖于另一方行为的变动时，可使用观察者模式松耦谷联动双方，使得一方的变动可以通知到感兴趣的另一方对象，从而让另一方对象对此做出响应。</p>
<p>观察者模式适用于以下几种应用场景：</p>
<p>1、当一个抽象模型包含两个方面内容，其中一个方面依赖于另一个方面；</p>
<p>2、其他一个或多个对象的变化依赖于另一个对象的变化；</p>
<p>3、实现类似广播机制的功能，无需知道具体收听者，只需分发广播，系统中感兴趣的对象会自动接收该广播；</p>
<p>4、多层级嵌套使用，形成一种链式触发机制，使得事件具备跨域（跨越两种观察者类型）通知。</p>
<h3 id="通用代码"><a href="#通用代码" class="headerlink" title="通用代码"></a>通用代码</h3><p>抽象观察者（Observer）</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">interface</span> <span class="title">IObserver</span>&lt;<span class="title">E</span>&gt;</span></span><br><span class="line"><span class="class"></span>&#123;</span><br><span class="line">    <span class="function"><span class="keyword">void</span> <span class="title">update</span><span class="params">(E e)</span></span>;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<p>具体观察者（ConcrereObserver）</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">ConcreteObserver</span>&lt;<span class="title">E</span>&gt; <span class="keyword">implements</span> <span class="title">IObserver</span>&lt;<span class="title">E</span>&gt;</span></span><br><span class="line"><span class="class"></span>&#123;</span><br><span class="line">	<span class="keyword">private</span> String name;</span><br><span class="line"></span><br><span class="line">	<span class="function"><span class="keyword">public</span> <span class="title">ConcreteObserver</span><span class="params">(String name)</span></span></span><br><span class="line"><span class="function">	</span>&#123;</span><br><span class="line">		<span class="keyword">this</span>.name = name;</span><br><span class="line">	&#125;</span><br><span class="line"></span><br><span class="line">	<span class="meta">@Override</span></span><br><span class="line">	<span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">update</span><span class="params">(E e)</span></span></span><br><span class="line"><span class="function">	</span>&#123;</span><br><span class="line">		System.out.println(name + <span class="string">"收到通知，更新状态:"</span> + e.toString());</span><br><span class="line">	&#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<p>抽象主题（Subject）</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">interface</span> <span class="title">ISubject</span>&lt;<span class="title">E</span>&gt;</span></span><br><span class="line"><span class="class"></span>&#123;</span><br><span class="line">    <span class="function"><span class="keyword">boolean</span> <span class="title">attach</span><span class="params">(IObserver&lt;E&gt; iObserver)</span></span>;</span><br><span class="line">    <span class="function"><span class="keyword">boolean</span> <span class="title">detach</span><span class="params">(IObserver&lt;E&gt; iObserver)</span></span>;</span><br><span class="line">    <span class="function"><span class="keyword">void</span> <span class="title">notify</span><span class="params">(E e)</span></span>;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<p>具体主题（ConcreteSubject）</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">ConcreteSubject</span>&lt;<span class="title">E</span>&gt; <span class="keyword">implements</span> <span class="title">ISubject</span>&lt;<span class="title">E</span>&gt;</span></span><br><span class="line"><span class="class"></span>&#123;</span><br><span class="line">	<span class="keyword">private</span> List&lt;IObserver&lt;E&gt;&gt; observerList = <span class="keyword">new</span> ArrayList&lt;&gt;();</span><br><span class="line"></span><br><span class="line">	<span class="meta">@Override</span></span><br><span class="line">	<span class="function"><span class="keyword">public</span> <span class="keyword">boolean</span> <span class="title">attach</span><span class="params">(IObserver&lt;E&gt; iObserver)</span></span></span><br><span class="line"><span class="function">	</span>&#123;</span><br><span class="line">		<span class="keyword">return</span> observerList.add(iObserver);</span><br><span class="line">	&#125;</span><br><span class="line"></span><br><span class="line">	<span class="meta">@Override</span></span><br><span class="line">	<span class="function"><span class="keyword">public</span> <span class="keyword">boolean</span> <span class="title">detach</span><span class="params">(IObserver&lt;E&gt; iObserver)</span></span></span><br><span class="line"><span class="function">	</span>&#123;</span><br><span class="line">		<span class="keyword">return</span> observerList.remove(iObserver);</span><br><span class="line">	&#125;</span><br><span class="line"></span><br><span class="line">	<span class="meta">@Override</span></span><br><span class="line">	<span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">notify</span><span class="params">(E e)</span></span></span><br><span class="line"><span class="function">	</span>&#123;</span><br><span class="line">		observerList.forEach(o -&gt; o.update(e));</span><br><span class="line">	&#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<p>测试代码</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">Main</span></span></span><br><span class="line"><span class="class"></span>&#123;</span><br><span class="line">	<span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(String[] args)</span></span></span><br><span class="line"><span class="function">	</span>&#123;</span><br><span class="line">		ConcreteObserver&lt;String&gt; concreteObserver1 = <span class="keyword">new</span> ConcreteObserver&lt;&gt;(<span class="string">"观察者1"</span>);</span><br><span class="line">		ConcreteObserver&lt;String&gt; concreteObserver2 = <span class="keyword">new</span> ConcreteObserver&lt;&gt;(<span class="string">"观察者2"</span>);</span><br><span class="line"></span><br><span class="line">		ConcreteSubject&lt;String&gt; concreteSubject = <span class="keyword">new</span> ConcreteSubject&lt;&gt;();</span><br><span class="line">		concreteSubject.attach(concreteObserver1);</span><br><span class="line">		concreteSubject.attach(concreteObserver2);</span><br><span class="line">		concreteSubject.notify(<span class="string">"subject"</span>);</span><br><span class="line"></span><br><span class="line">		concreteSubject.detach(concreteObserver1);</span><br><span class="line">		concreteSubject.notify(<span class="string">"new subject"</span>);</span><br><span class="line">	&#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<p>输出</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line">观察者1收到通知，更新状态:subject</span><br><span class="line">观察者2收到通知，更新状态:subject</span><br><span class="line">观察者2收到通知，更新状态:new subject</span><br></pre></td></tr></table></figure>



<h2 id="观察者模式在源码中的应用"><a href="#观察者模式在源码中的应用" class="headerlink" title="观察者模式在源码中的应用"></a>观察者模式在源码中的应用</h2><p>Spring中的 Context Loaderlistener实现了 Servletcontextlistener接口。</p>
<p>Servlet Contextlistener接口又继承了 Eventlistener，在JDK中 Eventlistener有非常广泛的应用。</p>
<p>可以看一下源代码，Contextloaderlistener</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">package</span> org.springframework.web.context;</span><br><span class="line"></span><br><span class="line"><span class="keyword">import</span> javax.servlet.ServletContextEvent;</span><br><span class="line"><span class="keyword">import</span> javax.servlet.ServletContextListener;</span><br><span class="line"></span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">ContextLoaderListener</span> <span class="keyword">extends</span> <span class="title">ContextLoader</span> <span class="keyword">implements</span> <span class="title">ServletContextListener</span> </span>&#123;</span><br><span class="line"></span><br><span class="line"></span><br><span class="line">	<span class="function"><span class="keyword">public</span> <span class="title">ContextLoaderListener</span><span class="params">()</span> </span>&#123;</span><br><span class="line">	&#125;</span><br><span class="line"></span><br><span class="line">	</span><br><span class="line">	<span class="function"><span class="keyword">public</span> <span class="title">ContextLoaderListener</span><span class="params">(WebApplicationContext context)</span> </span>&#123;</span><br><span class="line">		<span class="keyword">super</span>(context);</span><br><span class="line">	&#125;</span><br><span class="line"></span><br><span class="line">	<span class="comment">/**</span></span><br><span class="line"><span class="comment">	 * Initialize the root web application context.</span></span><br><span class="line"><span class="comment">	 */</span></span><br><span class="line">	<span class="meta">@Override</span></span><br><span class="line">	<span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">contextInitialized</span><span class="params">(ServletContextEvent event)</span> </span>&#123;</span><br><span class="line">		initWebApplicationContext(event.getServletContext());</span><br><span class="line">	&#125;</span><br><span class="line"></span><br><span class="line"></span><br><span class="line">	<span class="comment">/**</span></span><br><span class="line"><span class="comment">	 * Close the root web application context.</span></span><br><span class="line"><span class="comment">	 */</span></span><br><span class="line">	<span class="meta">@Override</span></span><br><span class="line">	<span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">contextDestroyed</span><span class="params">(ServletContextEvent event)</span> </span>&#123;</span><br><span class="line">		closeWebApplicationContext(event.getServletContext());</span><br><span class="line">		ContextCleanupListener.cleanupAttributes(event.getServletContext());</span><br><span class="line">	&#125;</span><br><span class="line"></span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>



<p>ServletContextListener 源码</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">package</span> javax.servlet;</span><br><span class="line"></span><br><span class="line"><span class="keyword">import</span> java.util.EventListener;</span><br><span class="line"></span><br><span class="line"></span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">interface</span> <span class="title">ServletContextListener</span> <span class="keyword">extends</span> <span class="title">EventListener</span> </span>&#123;</span><br><span class="line"></span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">default</span> <span class="keyword">void</span> <span class="title">contextInitialized</span><span class="params">(ServletContextEvent sce)</span> </span>&#123;</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">default</span> <span class="keyword">void</span> <span class="title">contextDestroyed</span><span class="params">(ServletContextEvent sce)</span> </span>&#123;</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<p>EventListener 就是一个空接口</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">/**</span></span><br><span class="line"><span class="comment"> * A tagging interface that all event listener interfaces must extend.</span></span><br><span class="line"><span class="comment"> * <span class="doctag">@since</span> JDK1.1</span></span><br><span class="line"><span class="comment"> */</span></span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">interface</span> <span class="title">EventListener</span> </span>&#123;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>



<h2 id="观察者模式的优缺点"><a href="#观察者模式的优缺点" class="headerlink" title="观察者模式的优缺点"></a>观察者模式的优缺点</h2><p>优点：</p>
<p>1、观察者和被观察者是松耦合（抽象耦合）的，符合依赖倒置原则；</p>
<p>2、分离了表示层（观察者）和数据逻辑层（被观察者），并且建立了一套触发机制，使得数据的变化可以响应到多个表示层上；</p>
<p>3、实现了一对多的通讯机制，支持事件注册机制，支持兴趣分发机制，当被观察者触发事件时，只有感兴趣的观察者可以接收到通知。</p>
<p>缺点：</p>
<p>1、如果观察者数量过多，则事件通知会耗时较长；</p>
<p>2、事件通知呈线性关系，如果其中一个观察者处理事件卡壳，会影响后续的观察者接收该事件； </p>
<p>3、如果观察者和被观察者之间存在循环依赖，则可能造成两者之间的循环调用，导致系统崩溃。</p>
 
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